In the fabrication of prior art gas laser tubes, the hermetic seal between each end of the tube and an associated optical element, such as a window, is accomplished by either means of thermal bonding or an epoxy gluing technique.
In the case of thermal bonding, the seal is made by holding the element and tube end in contact and raising the temperature to the melting point in the region of contact, as by r.f. induction or oven heating. The temperatures required for making such thermal bonds are on the order of at least several hundred degrees centigrade and have the concomitant effect of degrading the laser quality surface finish of the element, which in turn adversely affects light output during lasing operation. This problem can be avoided by using the epoxy gluing technique, but problems attendant with using epoxy also arise. For instance, the epoxy constituents tend to leak into the gas fill of the tube thereby causing contamination of the gas and seriously diminishing tube life. In addition, during tube useage the stability of operation with regard to lasing threshold, gain and power output progressively deteriorate due to the contamination. Another disadvantage of the epoxy technique is that it inhibits proper bakeout for removing contaminants during construction of the tube. It is generally desired that a bakeout be performed with the tube heated to a temperature of about 400.degree. centigrade while connected to a vacuum pump for evacuating water vapor and other contaminants which accumulated in the tube during manufacture. Epoxy has the characteristic, however, that it tends to become brittle and subject to breaking or cracking when heated above approximately 75.degree. centigrade. Moreover, heating of the epoxy to temperatures in excess of this level tends to accelerate the aforementioned contamination process associated with leaking of the epoxy constituents into the laser gas. In addition, the epoxy may degrade to the point at which it can no longer provide a leak tight seal. Other window assemblies have employed optically flat contacts, i.e., the window member is ground to an optical flatness and the tube to which it is to be joined is ground to an optical flatness and the two surfaces are pressed together to form a gas tight seal therebetween. In still other windows, the quartz or glass window member has been fused to a quartz or glass tubular extension of the envelope. In still another prior art embodiment the window member has been joined to the tubular envelope by means of a solder glass. The latter named techniques have one or more drawbacks associated therewith. For example, the optical window utilizing optical fits between abutting surfaces to form the seal cannot tolerate drastic temperature changes, and such optically flat contacting surfaces are generally expensive and difficult to produce. As to the prior art windows which have been fused to the envelope, these windows are typically relatively expensive to produce and require that a part of the window material be softened by heating during the joining process. This heating of the window creates strains which can alter the surface of the window and disturb the wave front and/or plane of polarization of the laser beam, thereby drastically reducing the power output obtained from the laser. Windows sealed to the envelope by means of solder glass are easier to fabricate than fused windows but suffer from strains which can alter the window surface and create disturbances of the wavefront or plane of polarization of the laser beam transmitted through the window.
Therefore, a need exists for a laser mirror assembly, the assembly comprising a mirror sealed to a metal frame member, which is capable of withstanding the relatively high bakeout temperatures utilized for evacuating contaminants from the fabricated laser tube, the seal additionally minimizing gas leakage therethrough during normal laser operation. This would eliminate the necessity of a specially designed laser optical element, as shown in U.S. Pat. No. 3,555,450, which allows grinding and polishing of the optical element after it has been assembled into the frame structure and subsequent to high temperature bakeout. Further, the laser mirror assembly should provide an essentially vacuum tight laser tube, be insensitive to thermal cycling within predetermined temperature ranges, not be effected by humidity, whereby laser tubes of relatively long shelf and operating lifetimes can be realized.